Distributed aperture automotive radar system with alternating master radar devices

ABSTRACT

A distributed aperture bi-static radar system, apparatus, architecture, and method is provided for coherently combining physically distributed radars to jointly produce target scene information in a coherent fashion by alternately selecting first and second small aperture devices to operate as the master unit so that radar signals are sequentially transmitted from every transmit antenna in the first and second small aperture devices, thereby enabling the radar control processing unit to coherently combine mono-static and bi-static virtual array apertures from the first and second small aperture radar devices to construct an extended bi-static MIMO virtual army aperture that is larger than the bi-static MIMO virtual array apertures produced by the first and second small aperture radar devices.

BACKGROUND OF THE INVENTION Cross-reference to Related Applications

U.S. patent application Ser. No. 16,356,764, entitled “DistributedAperture Automotive Radar System,” by inventors Ryan H. Wu and AruneshRoy, filed on even date herewith, describes exemplary methods andsystems and is incorporated by reference in its entirety.

U.S. patent application Ser. No. 16,356,792, entitled “High ResolutionAutomotive Radar System with Forward and Backward Difference Co-ArrayProcessing,” by inventor Ryan H. Wu, filed on even date herewith,describes exemplary methods and systems and is incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention is directed in general to radar systems andassociated methods of operation. In one aspect, the present inventionrelates to an automotive radar system formed with independentdistributed radars.

DESCRIPTION OF THE RELATED ART

Radar systems may be used to detect the range and velocity of nearbytargets. With advances in technology, radar systems may now be appliedin many different applications, such as automotive radar safety systems,but not every radar system is suitable for every application. Forexample, 77 GHz Frequency Modulation Continuous Wave (FMCW) Fast ChirpModulation (FCM) radars are used as primary sensors in Advanced DriverAssistance System (ADAS) and are used as safety sensors in autonomousdriving (AD) systems, but are not used as the primary sensor in ADsystems due to limited angular resolution performance. To enable the useof such radar systems as the primary sensor for driver replacement in ADsystems, such systems must provide better angular resolution, but thistypically requires larger antenna apertures, and therefore physicallylarger radars. Unfortunately, the requirement of having larger radarscan conflict with other design and/or operational constraints, such asintegrating a large aperture radar into a vehicle which has competingrequirements for design, structure, and/or operation. For example, thefront of a vehicle may have design or structural elements (e.g.,headlights, design emblems, bumpers, etc.) that do not readily admit theaddition of a large aperture radar. Keeping the size of radarsufficiently small so it can be integrated with other parts of thevehicle implies that the aperture of the radar is constrained and thusthe angular resolution is limited.

Existing radar systems have attempted to address these challenges byusing techniques (e.g., bistatic multiple-input multiple-output radar)which effectively combine a plurality of distributed, smaller apertureradars to form a larger virtual aperture. However, these techniquestypically require that the distributed radars share a common referencelocal oscillator (LO) signal (so the radars operate on the samefrequency and time reference) and/or require complex and expensivemodifications to hardware and software to cross-correlate or mix targetreturn signals with the signals from other transmitters. Unfortunately,these requirements may not be possible due to car integration,complexity, and/or cost constraints which prevent such solutions frombeing robustly and economically implemented. As seen from the foregoing,the existing radar system solutions are extremely difficult at apractical level by virtue of the challenges with achieving theperformance benefits of larger size radars within the performance,design, complexity and cost constraints of existing radar systemapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood, and its numerous objects,features and advantages obtained, when the following detaileddescription of a preferred embodiment is considered in conjunction withthe following drawings.

FIG. 1 is a simplified schematic diagram of a distributed coherent radarsystem in accordance with selected embodiments of the presentdisclosure.

FIGS. 2A-E are simplified diagrammatic depictions of the physical radarapertures and MIMO virtual array apertures provided by two distributedradars in accordance with selected embodiments of the presentdisclosure.

FIG. 3 depicts simulation normalized spatial frequency or angle spectrumof beamformer output results for three types of array processingexamples in accordance with selected embodiments of the presentdisclosure.

FIG. 4 depicts an enlarged view of the simulation angle spectrum shownin FIG. 3.

FIG. 5 illustrates a simplified flow chart showing the logic forcombining multiple distributed small-aperture radars to form a virtuallylarge coherent aperture in accordance with a first selected embodimentof the present disclosure.

DETAILED DESCRIPTION

A distributed aperture radar system, hardware circuit, system,architecture, and methodology are described for jointly producing targetscene information with multiple coherent radars that do not require ashared common local oscillator reference. In selected embodiments, asignal processing methodology and algorithm are disclosed forcontrolling a plurality of physically distributed, small aperture radarsin a bi-static or multi-static radar transceiver to determine thefrequency offset Δf₀ and phase offset Δφ between master and slaveradars, thereby avoiding the requirement(s) of a shared/common LO signaland/or physically mixing or correlating the received other transmitters'transmissions with received target returns. In other embodiments, adistributed aperture, bi-static radar system is disclosed forconstructing an extended multiple-input-multiple-output (MIMO) apertureby alternating the role of the master transmitting radar among radars,thereby providing a bi-static MIMO aperture that is larger than thecombined physical size of the distributed apertures for greatly improvedangular resolution performance. In yet other embodiments, a signalprocessing methodology and algorithm are disclosed for constructing andaccumulating mono-static and bi-static MIMO virtual array outputs andthen performing forward and backward difference co-array processing andconstruction for cascaded physical and virtual array processing tomitigate or suppress spurious sidelobes in the formed radar beampattern. By providing hardware and software solutions for combiningindependent distributed radars that are offset in both frequency andphase, the disclosed distributed aperture radar system and methodologyefficiently provide a coherent bi-static MIMO virtual array having anaperture that is many times larger than the total physical aperturescombined, thereby achieving better sensitivity, finer angularresolution, and low false detection rate. And by using the disclosedforward and backward single-frame difference co-array beamforming signalprocessing techniques, sparse apertures may be filled with virtualelements, resulting in very fine angular resolution while suppressingfalse detections due to spurious sidelobes.

In the context of the present disclosure, it will be appreciated thatradar systems may be used as sensors in a variety of differentapplications, including but not limited to automotive radar sensors forroad safety systems, such as advanced driver-assistance systems (ADAS)and autonomous driving (AD) systems. In such applications, the radarsystems are used to measure the radial distance to a reflecting objectand its relative radial velocity, and are characterized by performancecriteria, such as the angular resolution (the minimum distance betweentwo equal large targets at the same range and range rate (or radialvelocity) resolution cell which a radar is able to distinguish andseparate to each other), sensitivity, false detection rate, and thelike. Typically, frequency modulated continuous wave (FMCW) modulationtechniques are used to identify the distance and/or velocity of a radartarget, such as a car or pedestrian, by transmitting FMCW modulatedsignals from multiple transmit antennas so that reflected signals fromthe radar target are received at multiple receive antennas and processedto determine the radial distance and relative radial velocity anddirection for the radar target. However, with current automotivedesigns, a vehicle can include multiple radars which operateindependently from one another. Typically, one radar's transmissions arenot used by any other radars, and are instead treated as interferenceswhich need to be avoided or suppressed to prevent interference.Alternatively, the outputs from individual radars are used independentlyor integrated in a non-coherent fashion or fused by the tracker.Techniques for non-coherently combining multiple front-endsystem-on-chip devices are known in literature; see for example thefollowing references: P. Swirhun, “Millimeter-Wave Circuit Design forRadar Transceivers (2013); NXP Fact Sheet entitled “MR2001: 77 GHZ RadarTransceiver Chipset” (2015); and Texas Instruments publication entitled“AWR1642 mmWave sensor: 76-81-GHz radar-on-chip for short-range radarapplications” (2017). However, non-coherent integration or track fusiondoes not increase the system angular performance.

And while there are systems which combine distributed apertures to forma larger aperture, such systems typically require that the distributedradars share a common reference local oscillator (LO) signal so theradars operate on the same frequency and time reference. The commonreference LO signal is usually shared via physical wave-guideconnections (e.g., PCB transmission lines, substrate integratedwaveguides, coaxial cables, etc.) or even wireless connections that haveprecisely measured phase delays within the frequency range of operation.Techniques for combining multiple front-end system-on-chip devices witha shared, distributed LO signal are known in literature; see for examplethe following references: F. Starzer et al., “A Novel 77-GHz RadarFrontend with 19-GHz Signal Distribution on RF-PCB Substrate,” 2010Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems(SiRF), pp. 152-155 (2010); U.S. Patent Pub'n. No. 2016/0018511AI to J.Nayyar et al. entitled “Distributed Radar Signal Processing in a RadarSystem”; and “Texas Instruments Application Report, AWR1243 Cascade”(October 2017, revised December 2017). However, there are oftensituations where car integration constraints prevent such connectionsfrom being robustly and economically implemented.

As an alternative to physically sharing the LO signal, distributedapertures can also be combined in systems, such as bi-static radarsystems, that form a single large aperture by having each radar receivethe other radars' transmissions and then cross-correlate the targetreturns with the received transmissions from the other radars forestimating target parameters. Multi-static radar techniques are known inliterature, see for example the following references: U.S. Pat. No.3,487,462 to D. Holberg entitled “Bistatic radar configuration notrequiring reference-data transmission”; U.S. Pat. No. 3,812,493 to M.Afendykiw et al. entitled “Bistatic passive radar”; and U.S. Pat. No.4,994,809 to K. Tung et al. entitled “Polystatic correlating radar.”Such approaches, however, require relatively complex and expensivemodifications to existing automotive radar transceiver hardware andsoftware because of the lack of the dedicated cross-correlator circuitryfor the reference signal.

To address these limitations from conventional solutions and othersknown to those skilled in the art, reference is now made to FIG. 1 whichdepicts a simplified schematic diagram of a distributed coherent radarsystem 100 which includes two or more distributed radar devices 10, 20connected to a radar controller processor 30. In selected embodiments,each of the distributed radar devices 10, 20 may be embodied as aline-replaceable unit (LRU) or modular component that is designed to bereplaced quickly at an operating location. Similarly, the radarcontroller processor 30 may be embodied as a line-replaceable unit (LRU)or modular component. Although two distributed radar devices 10, 20 areshown, it will be appreciated that additional distributed radar devicesmay be used. In addition, the depicted radar system 100 may beimplemented in integrated circuit form with the distributed radardevices 10, 20 and the radar controller processor 30 formed withseparate integrated circuits (chips) or with a single chip, depending onthe application.

Each distributed radar device 10, 20 includes one or more transmittingantenna elements TX_(i) and receiving antenna elements RX connected,respectively, to one or more radio-frequency (RF) transmitter (TX) units11, 21 and receiver (RX) units 12, 22. For example, each radar device(e.g., 10) is shown as including individual antenna elements (e.g.,TX_(1,i), RX_(1,j)) connected, respectively, to three transmittermodules (e.g., 11) and four receiver modules (e.g., 12), but thesenumbers are not limiting and other numbers are also possible, such asfour transmitter modules 11 and six receiver modules 12, or a singletransmitter module 11 and/or a single receiver modules 12. Each radardevice 10, 20 also includes a chirp generator 112, 132 which isconfigured and connected to supply a chirp input signal to thetransmitter modules 11, 21. To this end, the chirp generator 112, 132 isconnected to receive a separate and independent local oscillator (LO)signal generator 110, 130 so that the distributed radars 10, 20 do notshare a common local oscillator (LO) signal, but are instead operated ina coordinated but non-coherent fashion as disclosed herein. In addition,a common chirp start trigger signal may be shared amongst the chirpgenerators 112, 132, though delays are likely to be different due to thesignal path differences and programmable digital delay elements 111,131. As a result, the transmitter elements 11, 12 operate in anon-coherent fashion because, even though they are programmed totransmit identical waveforms and share a common schedule, the generatedwaveforms are likely to have distinct starting frequencies, phases, andtransmitting time.

The radar system 100 also includes a radar controller processing unit 30that is connected to supply input control signals to the distributedradar devices 10, 20 and to receive therefrom digital output signalsgenerated by the receiver modules 12, 22. In selected embodiments, theradar controller processing unit 30 may be embodied as amicro-controller unit (MCU) or other processing unit that is configuredand arranged for signal processing tasks such as, but not limited to,target identification, computation of target distance, target velocity,and target direction, and generating control signals. The radarcontroller processing unit 30 may, for example, be configured togenerate calibration signals, receive data signals, receive sensorsignals, generate frequency spectrum shaping signals (such as rampgeneration in the case of FMCW radar) and/or state machine signals forRF (radio frequency) circuit enablement sequences. In addition, theradar controller processor 30 may be configured to program the modules(s) 11, 21 to operate in a coordinated fashion by transmitting MIMOwaveforms for use in constructing a virtual aperture from a combinationof the distributed apertures formed by the distributed transmitting andreceiving antenna elements TX_(i), RX_(j).

In the example shown, each chirp generator 112, 132 generates a chirpsignal in response to a chirp start trigger signal generated by thedelay circuit 111, 131, and a corresponding reference local oscillatorsignal LO #1, LO #2 generated by frequency synthesizer unit 10, 130.Since the reference LO signals are independent, they may have differentfrequency and phase values, which in turn affects the frequency andphase of the generated chirp signal. The resulting chirp signal fromeach generator 112, 132 is then processed by the RF conditioning unit113, 133 and amplified at the power amplifier (PA) 114, 134 whichamplifies the signal to a level suitable for transmission as a radarsignal by a transmitter antenna unit TX_(1,j), TX_(2,i). Though notshown, it will be understood that the transmitter module 11, 21 mayinclude additional processing circuits, such as a digital-to-analogconverter (DAC), phase shifter (or phase rotator), buffer, mixer,filter, and the like.

The radar signal transmitted by the transmitter antenna unit TX_(1,j),TX_(2,j) may by reflected by an object, such as a vehicle 1. Part of thereflected radar signal (e.g., mono-static target returns) reachesreceiver antenna units RX_(1,i) at the first distributed radar device10, and another part (e.g., bi-static target returns) reaches receiverantenna units RX_(2,j) at the second distributed radar device 20. Ateach receiver module 12, 22, the received (radio frequency) antennasignal is amplified by a low noise amplifier (LNA) 120, 140 and then fedto a mixer 121, 141 where it is mixed with the RF conditioned signalgenerated by the RF conditioning unit 113, 133. The resultingintermediate frequency signal is fed to a first high-pass filter (HPF)122, 142. The resulting filtered signal is fed to a first variable gainamplifier 123, 143 which amplifies the signal before feeding it to afirst low pass filter (LPF) 124, 144. This re-filtered signal is fed toan analog/digital converter (ADC) 125, 145 and is output by eachreceiver module 12, 22 as a digital signal D1, D2, etc.

In order for each receiver module 11, 21 to be able to distinguishtransmitted radar signals from reflected radar signals, the transmittedradar signals may be coded so they can be separated at the receivermodules 12, 22. Such separability can be achieved with code-divisionmultiple access (CDMA), frequency-division multiple access (FDMA), ortime-division multiple access (TDMA) techniques. For example, thetransmitter antenna units TX_(i) on each distributed radar device (e.g.,10) may be controlled and configured to transmit one at a time to form aTime-Division MIMO aperture. In another example, each transmitterantenna unit TX_(i) may be controlled and configured to transmit with anamount of frequency shift for forming Doppler-Division (DD) orFrequency-Division (FD) MIMO apertures. In yet another example, eachtransmitter antenna units TX_(i) may be controlled and configured totransmit all at once, but with chirps coded with pseudo-random sequencesthat are orthogonal across transmitters for forming Code-Division (CD)MIMO apertures.

Under control of the radar controller processor 30, the distributedcoherent radar system 100 is configured to operationally combinemultiple, physically separated small-aperture radars 10, 20 to functionas a single large coherent aperture radar. To this end, the radarcontroller processor 30 may be configured with a bi-static radar module31 which is operative to combine the distributed aperture signal resultswithout requiring physically mixing or correlating received targetreturns with the signals received from other transmitters. However, toachieve this result, the differences in the starting frequency and phasefor the reference LO signals must be determined before the system canfunction as a single radar by coordinating the distributed radar devices10, 20 to operate in a coherent fashion. Accordingly, the bi-staticradar module 31 includes a frequency/phase measurement module 37 thatproduces frequency offset measurements (Δf₀) and phase offsetmeasurements (Δφ). By using the frequency/phase measurement module 37 tocompute or measure the frequency and phase offsets between thedistributed radars 10, 20, one may compensate for the differences andthe different radar signals may then be processed in a coherent fashionas if it is a single radar. Note that the time offsets (Δt₀) amongstdistributed radars are assumed to be known which is measured at the timeof factory installation and system integration and testing.

In addition to the bi-static radar aperture construction, the radarcontroller processor 30 may be configured to construct and accumulatemultiple-input-multiple-output (MIMO) array outputs to form a MIMOaperture. To this end, the radar controller processor 30 may beconfigured with a MIMO virtual array module 38 which is operative toalternate the transmitting “master” role among the distributed radardevices 10, 20 so that an extended MIMO aperture can be formed based onMIMO radar principles. In operation, the MIMO virtual array module 38sequentially selects each of the distributed radar devices 10, 20 toserve as the “master” radar while the remaining radar devices operate as“slave” radar(s) until all of the distributed radar devices 10, 20 havebeen selected as the master unit once. The selected master-unit radardevice transmits radar waveforms and the slave-unit radar device(s)directionally receive and process the master radar's transmittedwaveforms using identical range and Doppler processing steps for normalradar waveforms. Applying the estimated frequency and phase offsets(Δf₀, Δφ) computed by the frequency/phase measurement module 37, eachslave radar produces coherent target measurements which the radarcontroller processor 30 uses to construct and accumulate mono-static andbi-static MIMO array outputs. The resulting bi-static MIMO aperture iseven larger than the combined physical size of the distributedapertures, which results in greatly improved angular resolutionperformance. If there are multiple sections of the resulting MIMO array,the MIMO virtual array module 38 may be configured to identify andselect the least-sparse section of the MIMO array to compute a first setof beamforming outputs.

As will be appreciated, the spacing and arrangement of the transmittingand receiving antenna elements TX_(i), RX_(j) may result in theconstruction of a sparse bi-static MIMO array (e.g., contains holes orgaps), resulting in high grating lobes in the formed radar beam pattern.To address the potential grating or spurious lobe issues of sparsearrays, the radar controller processor 30 may be configured with aco-array processing module 39 which is operative to perform forward andbackward difference co-array processing and cascaded physical and visualarray processing as a mitigation technique for suppressing the spurioussidelobes. In operation, the radar controller processor 30 uses theco-array processing module 39 to construct forward and backwarddifference co-array outputs based on the MIMO array outputs. If theformed difference co-array is uniformly spaced, the radar controllerprocessor 30 may perform spatial smoothing on the difference co-arrayoutputs. In addition or in the alternative, the radar controllerprocessor 30 may be configured to compute a second beamforming outputbased on the (spatially-smoothed) co-array outputs, and then compute acomposite beamforming output based on the first and the secondbeamforming outputs. Based on this processing, the radar controllerprocessor 30 may then generate and output the target Range-Doppler-Anglemap data.

Frequency/Phase Measurement

As indicated above, selected embodiments of the present disclosureprovide a method and apparatus for determining or measuring frequencyand phase differences between the distributed radar devices 10, 20 foruse in constructing a large, coherent virtual aperture radar frommultiple physically separated or distributed small-aperture radars bycompensating the distributed radars' radar measurements for processingin a coherent fashion. In the depicted distributed coherent radar system100, this is illustrated with the example of a radar controllerprocessor 30 which coordinates the coherent operation of two distributedradar devices 10, 20, each consisting of three transmit channels, fourreceiving channels, and an independent reference LO generator.

For illustration purposes, the first radar device 10 is selected as themaster unit to implement a time-division MIMO process wherein a firsttransmit antenna (TX_(1.1)) is radiating while the rest of the transmitantennas (TX_(1,2), TX_(1,3)) are not radiating. By design, the secondradar device 20 eavesdrops on the transmit signal from the firsttransmit antenna TX_(1.1) to receive the transmit signal and performfast-time and slow-time processing on the received signal in the sameway the second radar device 20 processes its own target returns. In FIG.1, this is shown with the second radar device which receives theeavesdropping propagation channel(s) at one or more of the receiveantennas (RX_(2,j)). After processing by the receiver modules 12, 22,the digital signals D1, D2 are each processed by the fast-time (range)FFT module 32 and slow-time (Doppler) FFT module 33, thereby generatingthe range-Doppler map (RDM). Because the eavesdropped signal is strong,it is easily identifiable from target returns in the range-Doppler map.

By premeasuring the propagation delays of the eavesdropping channels(e.g., at factory installation or during system integration andtesting), the range position of the (strongest) eavesdropping peak canbe found easily. As described more fully hereinbelow, thefrequency/phase measurement module 37 may be configured to use theDoppler and peak amplitude phase information of the (strongest)eavesdropping peak to derive the frequency offset Δf₀ and phase offsetΔφ between the transmitting radar 10 and the eavesdropping radar 20,respectively. If eavesdropping signals from multiple receivers areavailable, they can be coherently combined by compensating for thepre-measured phase differences between the receiving antennas of theeavesdropping propagation channels and then vectorially summed. This isequivalent to forming a directional receiving beam in the direction ofthe transmitting antenna (e.g., TX_(1.1)). Alternatively, a receiverchannel at the slave radar device 20 may be dedicated for eavesdroppingon the transmission of the master radar device 10 by connecting thereceiver channel to a directional antenna pointing at the master radardevice 10. This may be needed if the propagation environment dictateshigher gain.

While any suitable sequence of processing steps may be used by thefrequency/phase measurement module 37 to measure or compute thefrequency offset Δf₀ and phase offset Δφ, an example derivation sequenceis illustrated with reference to the distributed coherent radar system100 wherein the transmitter module (e.g., 11) in the first radar device10 is selected as the master radar to transmit with the transmit antennaTX_(1.1), and where the eavesdropping radar receivers (e.g., RX_(2.1),RX_(2.2), RX_(2.3), RX_(2.4)) in the second radar device 20 operate asslave radars. In this arrangement, the frequency offset (f₁-f₂) betweenthe slave and master radar may be denoted as Δf₀ (in Hz), the timeoffset (t₁-t₂) may be denoted as Δt₀ (in seconds), and the phase offset(φ₁-φ₂) between the radars may be denoted as Δφ (in radians). Withreference to the chirp generators 112, 132, the starting frequency ofthe linear-frequency modulation (LFM) chirp may be denoted as f₀ (inHz), and the chirping rate may be denoted as {dot over (f)} (in Hz/sec).Without loss of generality, it may be assumed that the initial phase andtransmitting time are both zeros, and the amplitude is “one.”

With this understanding, the received radar signal y_(ms)(t) at theoutput of the mixer 141 of the slave radar's receiver 21 can be modeledas shown below:

Master^(′)s  Chirp  at  Slave  Rx:${x_{m}(t)} = e^{j\; 2\;{\pi{({{({{\overset{.}{f}}_{0} + {\frac{1}{2}\overset{.}{f}\; t}})}t})}}}$Slave^(′)s  Own  Chirp:${x_{s}(t)} = {e^{j\;\Delta\;\varphi}e^{j\; 2\;{\pi{({{({f_{0} + {\Delta\; f_{0}} + {\frac{1}{2}{({\overset{.}{f} + {\Delta\;\overset{.}{f}}})}{({t + {\Delta\; t_{0}}})}}})}{({t + {\Delta\; t_{0}}})}})}}}}$Slave^(′)s  Mixer  output: $\begin{matrix}{{y_{m\; s}(t)} = {{{conj}( {x_{m}(t)} )}*{x_{s}(t)}}} \\{= {e^{j\;\Delta\;\varphi}e^{j\; 2\;{\pi{({{{({f_{0} + {\Delta\; f_{0}} + {\frac{1}{2}{({\overset{.}{f} + {\Delta\;\overset{.}{f}}})}{({t + {\Delta\; t_{0}}})}}})}{({t + {\Delta\; t_{0}}})}} - {{({f_{0} + {\frac{1}{2}\overset{.}{f}\; t}})}t}})}}}}} \\{= e^{j\; 2\;{\pi{({{{({{\Delta\; f_{0}} + {{({\overset{.}{f} + {\Delta\;\overset{.}{f}}})}\Delta\; t_{0}}})}t} + {\frac{1}{2}\Delta\;\overset{.}{f}\; t^{2}}})}}}} \\{e^{j\; 2\;{\pi{({{{({f_{0} + {\Delta\; f_{0}}})}\Delta\; t_{0}} + {\frac{1}{2}{({\overset{.}{f} + {\Delta\;\overset{.}{f}}})}\Delta\; t_{0}^{2}} + \frac{\Delta\;\varphi}{2\;\pi}})}}}}\end{matrix}$

The instantaneous frequency f_(ms)(t) at the mixer's output can beobtained by taking the time derivative of the received signal at themixer output y_(ms)(t), as shown below.

Instantaneous Frequency at Mixer Output:

$\begin{matrix}{{f_{m\; s}(t)} = {\frac{d}{dt}( \frac{\arg( {y_{m\; s}(t)} )}{j\; 2\;\pi} )}} \\{= {\frac{d}{dt}( {{( {{\Delta\; f_{0}} + {( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}}} )t} + {\frac{1}{2}\Delta\;\overset{.}{f}\; t^{2}} + {( {{\overset{.}{f}}_{0} + {\Delta\;{\overset{.}{f}}_{0}}} )\Delta\; t_{0}} +} }} \\ {{\frac{1}{2}( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}^{2}} + \frac{\Delta\;\varphi}{2\;\pi}} ) \\{= {( {{\Delta\; f_{0}} + {( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}}} ) + {\Delta\;\overset{.}{f}\; t}}}\end{matrix}$

As shown by the equation model f_(ms)(t), the instantaneous frequencyfor f_(ms)(t) varies linearly with time at a rate of Δ{dot over (f)}.Knowing that the instantaneous frequency f_(ms)(t) corresponds to thetime delay of the signal and knowing that the rate of the instantaneousfrequency f_(ms)(t) corresponds to the Doppler shift, it is seen thatthe value “Δ{dot over (f)}” is an observable quantity that can bedirectly derived from the Doppler position of the identifiedeavesdropped signal peak on the range-Doppler map (RDM) generated at theoutput of the slow-time (Doppler) FFT module 33. In FIG. 1, this isillustrated with the frequency/phase measurement module 37 receiving aninput from the slow-time (Doppler) FFT module 33 which generates therange-Doppler map. To obtain a higher accuracy estimate, the slow-time(Doppler) FFT module 33 may perform oversampling (e.g., zero-padding theslow-time samples for FFT) of the received radar signal.

Based on the first observation—that the value “Δ{dot over (f)}” can bedirectly derived from the Doppler position of the identifiedeavesdropped signal peak on the range-Doppler map (RDM)—it is seen thatthe equation for the instantaneous frequency model forf_(ms)(t)=(Δf₀+({dot over (f)}+Δ{dot over (f)})Δt₀)+Δ{dot over (f)}t isan observable quantity that can be estimated from fast-time FFT signalgenerated at the output of the fast-time (Range) FFT module 32. Inparticular, it corresponds to the range position of the eavesdroppedsignal's peak on the range-Doppler map (RDM) generated by the slow-time(Doppler) FFT module 33. In FIG. 1, this is illustrated with thefrequency/phase measurement module 37 receiving an input from theslow-time (Doppler) FFT module 33 which generates the range-Doppler map.As will be appreciated, a higher accuracy estimate can be obtained byperforming oversampling (e.g., zero-padding) at the fast-time (Range)FFT module 32. Since Δt₀ is a pre-measured known value, Δ{dot over (f)}is obtained from the Doppler position of the identified eavesdroppedsignal peak, and {dot over (f)} is the known chirp rate, it can be seenthat the value Δf₀ can be solved from the equation for the instantaneousfrequency model for f_(ms)(t).

Based on the derived equation for the mixer output model y_(ms)(t), itis seen that the instantaneous frequency equation term

$ {{( {f_{0} + {\Delta\; f_{0}}} )\Delta\; t_{0}} + {\frac{1}{2}( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}^{2}} + \frac{\Delta\;\varphi}{2\pi}} )$is an observable quantity which corresponds to the phase of the peakamplitude of the eavesdropped signal, and can be estimated by dividingthe fast-time FFT's phase with 2π. Given known and obtained values off₀, Δf₀, Δt₀, and {dot over (f)}, and assuming further that Δ{dot over(f)} is negligible (in fact, the entire ½({dot over (f)}+Δ{dot over(f)})Δt₀ ² is likely negligible), Δ{dot over (f)} can therefore besolved.

To measure or compute the frequency offset measurements (Δf₀) and phaseoffset measurements (Δφ) between the distributed radars, thefrequency/phase measurement module 37 is connected to receive theresults of the processing steps 32-36 in the bi-static radar module 31,either in the form of the target tracks (TRACKS) generated by the targettracking module 36 or directly from the intermediate processing stages,such as the fast-time (range) FFT module 32 and/or the slow-time(Doppler) FFT module 33. With these inputs, the frequency/phasemeasurement module 37 is also configured with control code and datastructures to represent the signal models for the mixer output signaly_(ms)(t) and instantaneous mixer output frequency f_(ms)(t) and toproduce the frequency offset measurements (Δf₀) and phase offsetmeasurements (Δφ) between the distributed radars. Applying the derivedfrequency and phase offset measurements as compensations to the slavemeasurements (or, alternatively, to the master measurements), the radarcontroller processor 30 effectively compensates for differences and theradars' signals so that they may be processed in a coherent fashion asif the two radars are a single coherent radar.

Bi-Static MIMO Virtual Array Aperture with Alternating Master Radars

As indicated above, selected embodiments of the present disclosureprovide a method and apparatus for constructing a bi-staticmultiple-input-multiple-output (MIMO) virtual array aperture frommultiple, distributed smaller apertures by alternating the role of themaster transmitting radar among radars, thereby providing virtual arrayaperture that is larger than the combined physical size of thedistributed, smaller apertures for greatly improved angular resolutionperformance. In the depicted distributed coherent radar system 100),this is illustrated with the example of a radar controller processor 30where the MIMO virtual array module 38 coordinates the coherentoperation of two distributed radar devices 10, 20 by alternating therole of the master radar between them.

As an initial step, the first radar device 10 is selected as the masterunit which implements a time-division MIMO process wherein a firsttransmit antenna (TX_(1,1)) is selected to transmit or radiate radarsignals while the rest of the transmit antennas (TX_(1,2), TX_(1,3)) arenot radiating. Receiver antennas RX_(1,1) to RX_(1,4) of the selectedmaster radar device 10 and receiver antennas RX_(2,1) to RX_(2,4) of theslave radar device 20 receive and process the mono-static and bi-statictarget returns, as shown. Subsequently, the second transmit antenna(TX_(1,2)) and third transmit antenna (TX_(1,3)) of the selected masterradar device 10 are sequentially selected to transmit or radiate radarsignals that are received and processed as target returns at thereceiver antennas RX_(1,1)-RX_(1,4) and RX_(2,1)-RX_(2,4). Based on thetarget returns from the master radar device 10 received at the receiverantennas RX_(1,1)-RX_(1,4) of the slave radar device 20, a firstmono-static MIMO virtual array aperture may be formed. In addition, asecond bi-static MIMO virtual array may be formed from the targetreturns from the master radar device 10 received at the receiverantennas RX_(2,1)-RX_(2,4) of the slave radar device 20. Together, themono-static MIMO aperture and bi-static MIMO aperture form a bi-staticMIMO virtual aperture consisting of antenna element positions being thevectoral sum of the transmit antenna element position and the receiverantenna element position.

To provide additional details for an improved understanding of selectedembodiments of the present disclosure, reference is now made to FIGS.2A-C which provide simplified diagrammatic depictions of the physicalradar apertures and resulting MIMO virtual array apertures which may begenerated by distributed radar devices 201, 202 in accordance withselected embodiments of the present disclosure. In particular, FIG. 2Aillustrates a distributed arrangement 200A of first and second radardevices 201, 202, each having three transmit antennas and three receiverantennas which are symmetrically positioned and distributed in relationto one another. In particular, the first distributed radar device 201includes three physical transmit antennas T_(1,1)-T_(1,3), with threephysical receiver antennas R_(1,1)-R_(1,3) positioned between the firstand second physical transmit antennas T_(1,1), T_(1,2). In the depictedexample of a mirrored arrangement, the second distributed radar device202 includes three physical transmit antennas T_(2,1)-T_(2,3), withthree physical receiver antennas R_(2,1)-R_(2,3) positioned between thesecond and third physical transmit antennas T_(2,2), T_(2,3). Withoutloss of generality, the physical antennas are shown as being positionedin a linear fashion, but may be arranged in non-linear fashion.

To illustrate an example virtual array aperture that may be formed withthe distributed radar devices 201, 202 when the first radar device 201is selected as the master radar, reference is now made to FIG. 2B whichillustrates a first MIMO virtual array aperture 200B formed bytransmitting radar signals from the three transmitting antennasT_(1,1)-T_(1,3) of the first radar device 201 which are received at thereceiving antennas R_(1,1)-R_(1,3), R_(2,1)-R_(2,3) from both radardevices 201, 202. On the left side, the mono-static MIMO virtual arrayelements 204 are generated by the receiving antennas R_(1,1)-R_(1,3) onthe master radar device 201 which receive radar transmit signals whichare sequentially radiated by the three transmitting antennasT_(1,1)-T_(1,3) of the master radar device 201. On the right side, thebi-static MIMO virtual array elements 205 are generated by the receivingantennas R_(2,1)-R_(2,3) on the slave radar device 202 which receiveradar transmit signals which are sequentially radiated by the threetransmitting antennas T_(1,1)-T_(1,3) of the master radar device 201. Asa result, the first MIMO virtual array aperture 200B has more elementsthan the physical array 200A and occupies a larger (wider) area. Sinceangular resolution is inversely proportional to aperture size, the MIMOvirtual array aperture 200B provides improved angular resolution (ascompared to the physical array 200A). However, it is also seen that thefirst MIMO virtual array aperture 200B is a “sparse” array whichcontains holes or gaps between the virtual array elements.

In accordance with selected embodiments of the present disclosure, thesize of the MIMO virtual array aperture may be increased further by alsoselecting the second radar device 202 to operate as the master unit sothat the first radar device 201 operates as the slave unit. In thisarrangement where the second radar device 202 is selected as the masterunit and the first radar device 201 is selected as the slave unit, thethree transmitting antennas T_(2,1)-T_(2,3) of the second (master) radardevice 202 are sequentially used to generate target returns at thereceiving antennas R_(1,1)-R_(1,3), R_(2,1)-R_(2,3) from both radardevices 201, 202. Where there is no shared or common reference LO signalfor the radar devices 201, 202, the ability to switch the “master unit”role between radar devices 201, 202 requires that the frequency andphase offset measurement values be obtained and applied to compensatethe slave (or reversely to the master's) measurements and thereby enablecoherent processing of the combined target returns from the radardevices 201, 202.

After using both radar devices 201,202 as “master” units to transmitradar signals from all of the transmit antennas T_(1,1)-T_(1,3),T_(2,1)-T_(2,3), a second MIMO virtual array aperture 200C may beformed, as shown in FIG. 2C. As depicted, the second MIMO virtual arrayaperture 200C includes a first set of mono-static MIMO virtual arrayelements 206 generated by the receiving antennas R_(1,1)-R_(1,3) whenreceiving sequentially transmitted radar transmit signals from thetransmitting antennas T_(1,1)-T_(1,3) of the (master-designated) firstradar device 201. In addition, the second MIMO virtual array aperture200C includes a second set of mono-static MIMO virtual array elements208 generated by the receiving antennas R_(2,1)-R_(2,3) when receivingsequentially transmitted radar transmit signals from the transmittingantennas T_(2,1)-T_(2,3) of the (master-designated) second radar device202. Finally, the second MIMO virtual array aperture 200C includes athird set of bi-static MIMO virtual array elements 207 generated by thereceiving antennas R_(1,1)-R_(1,3), R_(2,1)-R_(2,3) when receivingsequentially transmitted radar transmit signals from the transmittingantennas T_(1,1)-T_(1,3), T_(2,1)-T_(2,3) of the radar device 201. Asindicated with the grouping box 209, the third set of bi-static MIMOvirtual array elements 207 includes redundant or overlappingcontributions from the first and second radar devices when acting intheir respective master unit roles. By virtue of alternating the masterrole amongst the distributed radar devices 201, 202, the second MIMOvirtual array aperture 200C has more elements than the virtual array200B and occupies a larger (wider) area since it is based on all sixtransmit antennas T_(1,1)-T_(1,3), T_(2,1)-T_(2,3) and receiver antennasR_(1,1)-R_(1,3), R_(2,1)-R_(2,3). As a result, the larger second MIMOvirtual array aperture 200C provides improved angular resolution ascompared to the first MIMO virtual array aperture 200B. However, it isalso seen that the second MIMO virtual array aperture 200C is still a“sparse” array which contains holes or gaps between the virtual arrayelements, though the third set of bi-static MIMO virtual array elements207 is the least-sparse contiguous array section.

While the second MIMO virtual array aperture 200C is described withreference to alternating the master radar role between two distributedradars 201, 202, it will be appreciated that the principle can bereadily extended to additional radars. In addition, the benefits ofalternating master radar transmissions can be extended to non-LFM(Linear Frequency Modulated) chirp radar, as well as to other forms ofMIMO besides TD-MIMO.

Forward and Backward Difference Co-Array Processing

As indicated above, the distributed nature of the physical apertures tobe combined (e.g., 201, 202) result in the formation of larger MIMOvirtual array apertures (e.g., 200C) which are likely sparse (i.e.,contains holes or gaps) and not entirely filled by virtual antennaelements by virtue of the Nyquist sampling requirements, and theresulting formed beams contain spurious side lobes because of undersampling and/or non-uniform sampling in the spatial domain. The presenceof grating lobes, or spurious side lobes in general, increase thelikelihood of false target detections in the angular domain.

To address these limitations and others known to those skilled in theart, selected embodiments of the present disclosure provide a signalprocessing apparatus, methodology and algorithm for constructing andaccumulating mono-static, bi-static, and multi-static MIMO virtual arrayoutputs and then performing forward and backward difference co-arrayprocessing and construction for cascaded physical and virtual arrayprocessing to mitigate or suppress spurious sidelobes in the formedradar beam pattern. In the depicted distributed coherent radar system100, this is illustrated with the example of a radar controllerprocessor 30 where the co-array processing module 39 constructsmono-static and bi-static MIMO virtual apertures, and then performsforward and backward difference co-array construction to mitigate thepotential grating lobe or spurious lobe issue of sparse arrays. Thegenerated co-array output may be further weighted with a windowingfunction to suppress the sidelobes in the formed beam pattern. Inaddition, the generated array beam pattern may be further weighted withthe beam pattern formed by a contiguous or less-sparse section of theMIMO virtual aperture to further suppress the spurious lobes.

While any suitable sequence of processing steps may be used by theco-array processing module 39 to perform forward and backward differenceco-array processing to mitigate the spurious sidelobes due to spatialunder and non-uniform sampling, an example processing sequence isillustrated with reference to the MIMO virtual array aperture 200C whichis further processed to generate the MIMO virtual array aperture 200Dshown in FIG. 2D. In particular and as described more fully hereinbelow,the forward and backward difference co-array is first constructed andthen the receive beam is formed based on the outputs of the constructedvirtual array. For each range-Doppler cell to be processed, its antennaoutputs across the formed MIMO array are further processed according tothe following steps. Without loss of generality, linear array withequally spaced spatial samples are assumed.

With reference to the MIMO virtual array aperture 200C shown in FIG. 2C,the i-th MIMO virtual array antenna element's position may be denoted asx_(i)=n_(i)*d, where d is the unit element spacing in meters and n_(i)is an integer. Ideally, d should be half wavelength for sampling theentire 180-degree field of view without ambiguity. In practice, theantenna's field of view is smaller than 180 degrees, so a larger spacingthan half wavelength may be used.

The forward and backward difference co-array construction starts byconstructing element pairs with respect to difference element-pairspacing. In an example where there are four antenna elements where [x₁,x₂, x₃, x₄]=[1, 2, 3, 5]*d, then the co-array virtual element is denotedx_(i,j)=x_(i)−x_(j).

For construction of the forward difference co-array, all combinationsresulting in zero or positive difference spacing are listed as theexample below:

-   -   x_(1,1)=0    -   x_(2,1)=d    -   x_(3,1)=2d    -   x_(4,1)=4d    -   x_(2,2)=0    -   x_(3.2)=d    -   x_(4,2)=3d    -   x_(3,3)=0    -   x_(4,3)=2d    -   x_(A,A)=0

Upon grouping the antenna pair indices by non-negative differencespacing values, the following list is constructed:

Difference Co-Array Element Spacing Antenna Pair Indices 0 x_(1, 1),x_(2, 2), x_(3, 3), x_(4, 4) d x_(2, 1), x_(3, 2) 2d x_(3, 1), x_(4, 3)3d x_(4, 2) 4d x_(4, 1)

The pair-wise difference operation indicates that a difference co-arrayaperture of the size of five (5) elements can be constructed. The formedelement outputs shall be calculated as follows.

First, the i-th MIMO antenna output is denoted as y_(i), which is thek-th difference co-array element's output. Based on antenna pair indices{x_(i1,j1), . . . , x_(iM,jM)}, the k-th difference co-array elementshould be calculated as

$z_{k} = {\frac{1}{M}{\sum\limits_{m = 1}^{M}{y_{im}{y_{jm}^{*}.}}}}$The resulting forward difference co-array element output is providedbelow:

Difference Co-Array Element Antenna Spacing Pair Indices Virtual elementoutput 0 x_(1,1), x_(2,2), x_(3,3), x_(4,4)$z_{0} = {\frac{1}{4}( {{y_{1}y_{1}^{*}} + {y_{2}y_{2}^{*}} + {y_{3}y_{3}^{*}} + {y_{4}y_{4}^{*}}} )}$D x_(2,1), x_(3,2)$z_{1} = {\frac{1}{2}( {{y_{2}y_{1}^{*}} + {y_{3}y_{2}^{*}}} )}$2d x_(3,1), x_(4,3)$z_{2} = {\frac{1}{2}( {{y_{3}y_{1}^{*}} + {y_{4}y_{3}^{*}}} )}$3d x_(4,2) z₃ = y₄y₂ ^(*) 4d x_(4,1) z₄ = y₄y₁ ^(*)

As seen from above, the outputs {z₀, z₁, . . . z₄} are then used as theoutputs corresponding to an antenna array with element positions (0, d,2d, 3d, 4d). Angle processing, such as beamforming, can then be carriedout.

In similar fashion, the backward difference co-array may be constructedby denoting x_(ij)=x_(i)−x_(j) for forward difference co-array, and thenselecting all combinations resulting in non-positive difference spacingto calculate and the outputs based on the same principle. Continuingwith the previous example, the pair indices and outputs of the elementsof the backward difference co-array are identified as the followingbackward difference co-array element output table:

Difference Co-Array Element Antenna Spacing Pair Indices Virtual elementoutput −4d x_(1,4) z⁻⁴ = y₁y₄ ^(*) = z₄ ^(*) −3d x_(2,4) z⁻³ = y₂y₄ ^(*)= z₃ ^(*) −2d x_(1,3), x_(3,4)$z_{- 2} = {{\frac{1}{2}( {{y_{1}y_{3}^{*}} + {y_{3}y_{4}^{*}}} )} = z_{2}^{*}}$−d x_(1,2), x_(2,3)$z_{- 1} = {{\frac{1}{2}( {{y_{1}y_{2}^{*}} + {y_{2}y_{3}^{*}}} )} = z_{1}^{*}}$0 x_(1,1), x_(2,2), x_(3,3), x_(4,4)$z_{0} = {\frac{1}{4}( {{y_{1}y_{1}^{*}} + {y_{2}y_{2}^{*}} + {y_{3}y_{3}^{*}} + {y_{4}y_{4}^{*}}} )}$

As seen from above, the virtual element output can be derived from theforward difference co-array outputs by taking the complex conjugate.

The final aperture is constructed based on the combined forward andbackward difference co-arrays. In this example, a virtual array of nine(9) elements is formed with antenna positions {−4d, −3d, −2d, −d, 0, d,2d, 3d, 4d}. Note that, if FFT is used for estimating target angles andthere are any uniform linear array positions missing an output, zerofilling should be performed to provide outputs for the missing arraypositions. Also note that, while conventional approaches for designingdifference co-arrays (e.g., the minimum redundancy array (MRA)technique) seek to minimize the redundancy by maximizing the forwarddifference co-array without any holes, selected embodiments of thepresent disclosure seek to maintain some redundancy since the averagingeffect between the overlapping contributions is helpful for reducingspurious sidelobes. In this way, a balance may be achieved between thedesign objectives of creating a large aperture and an evenly spreadredundancy. In selected embodiments, every virtual antenna array elementresults from an equal number of averaging contributions.

To provide additional details for an improved understanding of selectedembodiments of the present disclosure, reference is now made to FIGS.2D-E which provide simplified diagrammatic depictions of different MIMOvirtual array apertures 200D, 200E that may be generated by twodistributed radar devices 201, 202 in accordance with selectedembodiments of the present disclosure. In particular, FIG. 2Dillustrates a third MIMO virtual array aperture 200D in which a forwarddifference co-array is formed on top of the third MIMO virtual arrayaperture 200C formed with mono-static and bi-static MIMO virtual arrays206-208. With the third MIMO virtual array aperture 200D, the size ofthe aperture remains unchanged, but the aperture is fuller (e.g., fewerholes), resulting in lower spurious sidelobes. In FIG. 2E, a fourth MIMOvirtual array aperture 200E in which a forward and backward differenceco-array is formed on top of the mono-static and bi-static MIMO virtualarrays 206-208. With the fourth MIMO virtual array aperture 200E, thesize of the aperture is almost doubled (as compared to the third MIMOvirtual array aperture 200D) and the aperture is fuller, resulting inimproved angular resolution and improved spurious side lobe performance.

While the difference co-array processing techniques disclosedhereinabove improve the angular resolution and reduce the spurious sidelobes, there may be additional need for suppressing the spurious sidelobes. To this end, the co-array processing module 39 may be configuredto further reduce the spurious side lobes by spatially smoothing theforward/backward difference co-array element outputs in the forwarddirection. As will be appreciated, spatial smoothing is a technique usedin array signal covariance matrix construction for the purpose ofincreasing the matrix rank as well as decorrelating coherent signals. Asdisclosed herein, spatial smoothing may be used for improving thearrival signal's progressive phase change measurement by averaging outerror contributions. The co-array processing module 39 may beoperatively configured to define a size of the spatially smoothedaperture size. If the smoothed aperture is of the same size as theoriginal aperture size, no spatial smoothing is performed. However, ifthe smoothed aperture size is smaller, then a sliding-window averagingoperation is taken to produce the averaged outputs. Note that thespatial smoothing requires a virtual array of equally spaced antennaelements. If the virtual array does not have equally spaced antennaelements, this method does not apply.

In addition or in the alternative, the co-array processing module 39 maybe configured to further reduce the spurious side lobes by producing acomposite beam forming output. To this end, the co-array processingmodule 39 may be configured to multiply the beam forming output of theforward/backward difference co-array (with or without spatial smoothingapplied) with the beam forming output of a section of the MIMO virtualarray. The selected MIMO virtual array section ideally should not beunder-sampled (e.g., a section that forms a uniform linear array). Inlieu of a filled section of army, a section that is least sparse (i.e.,a few holes allowed) should be selected (e.g., section 207 indicated inFIG. 2C). By doing so, the spurious sidelobes in the output of thedifference co-array process are greatly suppressed.

To illustrate the improved side lobe suppression benefits of thedifferent array processing techniques disclosed herein, reference is nowmade to FIGS. 3-4 which depict a first view (FIG. 3) and enlarged view(FIG. 4) of simulation of normalized spatial frequency or angle spectrum(hereinafter, angle spectrum) of beamformer output results for threetypes of array processing examples for resolving three closely-spacedtargets in accordance with selected embodiments of the presentdisclosure. In the simulation, the locations of the three targets isshown, respectively, with the true target angle or spatial frequencies(corresponding to target directions) 301-303. In addition, thebeamforming angle spectrum 304 is generated using Discrete FourierTransform (DFT) of a single radar MIMO array, such as the array outputof the MIMO virtual array of a single radar aperture. An example singleradar array would be the mono-static MIMO virtual array 204 of FIG. 2B.

With the angle spectrum 305, there is shown the beamforming outputgenerated using DFT of the dual radar MIMO array output. In an exampleembodiment, the angle spectrum 305 is generated using DFT of the MIMOvirtual array of two distributed radars that are combined by alternatingthe master role between radar devices, such as the bi-static MIMOvirtual array 200C of FIG. 2C.

With the angle spectrum 306, there is shown the beamforming outputgenerated using Fast Fourier Transform (FFT) of the dual radar MIMOarray output with a forward-backward difference co-array. In an exampleembodiment, the angle spectrum 306 is generated using FFT of the virtualoutput of a MIMO Forward/Backward Difference Co-array of two distributedradars, such as the bi-static MIMO virtual array 200E of FIG. 2E.

The simulation angle spectrums 304-306 demonstrate the high angularresolution performance of the distributed aperture automotive radarsystems and methodologies disclosed herein. With the angle spectrum 304for a MIMO beamforming output of a single radar, the three targetscannot be resolved. This failure arises from the fact that the truetarget spatial frequencies 301-303 for the three targets are allcontained in a single broad main central lobe instead of three distinctmain lobes at the angle positions of the three targets.

In contrast, the angle spectrum 305 for a MIMO beamforming output ofdual radars shows that the three targets can be resolved when two radarsare combined in a coherent and alternating fashion when forming the MIMOaperture. This is seen in FIG. 4 where the angle spectrum 305 includesindividual lobes 305A-C which, respectively, contain the true spatialfrequencies 301-303 for the three targets. However, the angle spectrum305 also includes large spurious side lobes 305D-F which result in ahigher false target detection rate.

In the waveform 306 based on the forward and backward differenceco-array virtual array output, the spurious side lobes are suppressed.This is seen in FIG. 4 where the angle spectrum 306 includes individuallobes 306A-C which, respectively, contain the true spatial frequencies301-303 for the three targets, and also includes suppressed spuriousside lobes (e.g., 306D-E).

As will be appreciated from the foregoing, without coherently combiningthe radars, distributed radar outputs can only be combined in anon-coherent fashion which results in no impact in the size of apertureand it only improves the SNR or sensitivity performance of the radar.However, coherent combination of the radars not only improves theangular resolution, but also improves the SNR performance. In addition,it is noted that the principle of forward/backward difference co-arrayvirtual array processing can be applied to any physical or virtualarrays, and is not limited to MIMO virtual arrays.

To provide additional details for an improved understanding of selectedembodiments of the present disclosure, reference is now made to FIG. 5which depicts a simplified flow chart 500 showing the logic forcombining multiple distributed small-aperture radars to form a virtuallylarge coherent aperture. In an example embodiment, the control logic andmethodology shown in FIG. 5 may be implemented as hardware and/orsoftware on a host computing system, processor, or microcontroller unitthat includes processor and memory for storing programming control codefor constructing and operating a large virtual aperture radar bycoherently combining distributed small aperture radars which do notshare a common local oscillator signal.

The process starts (step 501), such as when a new radar frame isstarted. In radar systems, the transmitted radar signal may beperiodically modulated, such as by applying a frequency and/or phaseshift. The period is typically chosen such that the radar signalmodulation occurs between two time frames of the signal, where a timeframe may, for example, correspond with a “chirp” in an FMCW (frequencymodulation continuous wave) signal.

At step 502, one of the distributed radars is selected or designated asthe master unit, and any remaining distributed radar is selected ordesignated as a slave unit. The result of this selection is that, whenthe selected master-unit unit transmits on any transmit antenna, theother slave unit(s) turn off their transmit antennas and operate only inreceiver mode. As the process iteratively repeats itself through steps504-508 as described below, the processing at step 502 sequentiallyalternates the master unit role amongst the distributed radars byselecting a new master radar at each iteration until all of the radarshave been selected to operate as the master unit. In each iteration, theselected master-unit radar may sequentially transmit radar waveformsfrom each transmit antenna on the master unit radar, and the targetreturns from master radar's transmitted waveforms are directionallyreceived at the receiver antenna(s) of the master radar and also at thereceiver antenna(s) of the designated slave-unit radar(s). To initiateoperations at the distributed master and slave units, a trigger may besent to all units to signal the start of a chirp.

At step 504, each slave-unit radar processes the master radar'stransmitted waveforms by applying predetermined radar signal processingsteps to the radar waveforms received at each slave unit's receiverantenna(s). While any suitable radar signal processing steps may beused, each slave-unit radar may be configured to perform the samefast-time and slow-time processing on the received radar signal in thesame way it processes its own target returns range and Dopplerprocessing steps for the radar waveforms received at each slave unit'sreceiver antenna(s). For example, the slave-unit radar may apply rangeand Doppler FFT processing to generate mono-static and bi-static rangeDoppler maps.

At step 506, each slave-unit radar (or the radar controller) calculatesor estimates frequency and phase offset values, and then applies theoffsets to compensate for frequency and phase differences between themaster-unit radar and slave-unit radar. While any suitable estimationtechnique may be used to calculate the chirp starting frequency offset(Δf₀) and master-slave phase offset (Δφ), selected embodiments of thepresent disclosure configure each slave-unit radar may to estimate thesevalues based on the differences between the estimated range and Dopplermeasurements with the known truth. More specifically, a signalprocessing algorithm is implemented with software instructions which areexecuted to directly derive the frequency offset (Δf₀) from the Dopplerposition of the identified eavesdropped signal peak on the range-Dopplermap. In addition, the master-slave phase offset (Δφ) may be solved fromthe instantaneous frequency model:

${{f_{m\; s}(t)} = {\frac{d}{dt}( {{( {{\Delta\; f_{0}} + {( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}}} )t} + {\frac{1}{2}\Delta\; f\; t^{2}} + {( {f_{0} + {\Delta\; f_{0}}} )\Delta\; t_{0}} + {\frac{1}{2}( {\overset{.}{f} + {\Delta\;\overset{.}{f}}} )\Delta\; t_{0}^{2}} + \frac{\Delta\;\varphi}{2\pi}} )}},$based on the range-FFT estimated value off f_(ms)(t) and the known ornegligible values of f₀, Δf₀, Δt₀, {dot over (f)}, and Δ{dot over (f)}.Once the frequency and phase offset values are derived for eachslave-unit radar, the slave radars apply the estimated frequency andphase offsets to produce coherent target measurements. In this way, eachslave-unit radar (or radar controller) processes the slave-receivedmaster signal data to estimate the slave unit's frequency and phaseoffsets to the master unit's, thereby allowing the slave-unit radar togenerate coherent target data samples that are correlated in time,frequency, and phase with the master-unit radar.

At step 508, the correlated target return data samples received from thedistributed radar devices are processed using bi-static radar principlesto construct and accumulate mono-static and bi-static MIMO virtual arrayoutputs by combining the distributed apertures, but without requiringphysically mixing the received master-unit's transmissions with receivedtarget returns or sharing a local oscillator signal. In selectedembodiments, the processing of correlated target return data samplesgenerated by each master-unit is performed at the radar controllerprocessor to generate a bi-static MIMO virtual array which includes amono-static MIMO virtual array elements (e.g., 204) and bi-static MIMOvirtual array elements (e.g., 205).

At step 510, the process determines if all of the distributed radarshave been designated to operate as the master-unit radar. If not(negative outcome to detection step 510), then the process returns tostep 502 to select another one of the distributed radars as the newmaster-unit, and step 504-510 are repeated until all radars have beenselected as the master unit once. However, if the last radar has beenselected as a master-unit (affirmative outcome to detection step 510),then the process may proceed to perform additional processing.

At step 512, the accumulated MIMO virtual array outputs generated atstep 508 are processed to generate a first set of beamforming outputsthat define an extended MIMO virtual aperture based on MIMO radarprinciples. In selected embodiments, the processing of MIMO virtualarray outputs is performed at the radar controller processor to generatea first set of beamforming outputs using a selected least-sparse sectionof the MIMO virtual array.

At step 514, the MIMO array outputs are used to construct forwarddifferent co-array outputs, alone or in combination with backwarddifference co-array outputs. In selected embodiments, the forwarddifference co-array outputs are constructed by the radar controllerprocessor which determines relative distance positions of the antennaelements in the MIMO array, identifies all combinations of antennaelement pairs x_(i), x_(j)=x_(i)−x_(j) having a zero or positivedifference spacing, and then calculates virtual forward co-array elementoutputs. If desired, angle processing, such as beamforming, can then becarried out based on the forward difference co-array construction,resulting in a bi-static MIMO forward difference virtual co-arrayaperture (e.g., 200D) that is the same size as, but less sparse than,the bi-static MIMO virtual array aperture (e.g., 200C). In addition, theradar controller processor may construct the backward differenceco-array outputs by identifying all combinations of antenna elementpairs x_(i), x_(j)=x_(i)−x_(j) having a zero or negative differencespacing and then calculating virtual backward co-array element outputs.If desired, angle processing can then be carried out based on theforward and backward difference co-array construction, resulting in abi-static MIMO forward/backward difference virtual co-array aperture(e.g., 200E) that is larger than, and less sparse than, the bi-staticMIMO virtual array aperture (e.g., 200C). In selected embodiments, thebi-static MIMO forward/backward difference virtual co-array aperture isalmost twice as large as the bi-static MIMO virtual array aperture.

At step 516, additional processing is applied to suppress or reducespurious side lobes by performing spatial smoothing on theforward/backward difference co-array outputs if the MIMO array isuniformly spaced. In selected embodiments, the radar controllerprocessor may be configured to spatially smooth the forward/backwarddifference co-array element outputs in the forward direction. However,if the formed virtual array does not have equally spaced antennaelements, then step 516 is skipped.

At step 518, the (spatially smoothed) forward/backward differenceco-array outputs are processed to generate a second set of beamformingoutputs that define an extended MIMO virtual aperture based on MIMOradar principles. In selected embodiments, the processing of theforward/backward difference co-array outputs is performed at the radarcontroller processor. The size of the resulting aperture is almostdoubled and the aperture is fuller, which results in improved angularresolution and improved spurious sidelobe performance.

At step 520, additional processing is applied to suppress or reducespurious side lobes by computing composite beamforming outputs. Inselected embodiments, the radar controller processor may be configuredto produce a composite beam forming output by multiplying the beamforming output of the forward/backward difference co-array (with orwithout spatial smoothing applied) with the beam forming output of asection of the MIMO virtual array. By doing so, the spurious sidelobesin the output of the difference co-array process are greatly suppressed.

As will be appreciated by persons skilled in the art, the computeddifference co-array output can be further processed using anysuper-resolution angle estimation algorithms, included but not limitedto the beamforming algorithms based on Fourier analysis of the spatialfrequency components of the co-array outputs, such as Discrete FourierTransform or Fast Fourier Transform. Alternative super-resolution angleestimation algorithms include, but not limit to, Multiple SignalClassification (MUSIC) algorithm and its derivatives, RotationalInvariance (ESPRIT) algorithm and its derivatives, Matrix Pencilalgorithm and its derivatives, Method of Direction Estimation (MODE)algorithm, Noise or Signal Subspace Fitting algorithm or itsderivatives, Maximum Likelihood Estimator based algorithms, and SparsityConstraint based or L1-Norm minimization based algorithms, among others.

In addition, it will be understood that the forward and backwarddifference co-array processing can be applied to a distributed radarsystem whose apertures are physically separated but share a common LOsignal via a physical link. In this case, the eavesdropping processingsteps are removed. In addition, the forward and backward differenceco-array processing can be applied to a single radar system whose eitherphysical antenna array or virtual MIMO array is sparse. In this case,the processing steps reduce to mono-static case and the need for makingtwo distributed radar coherent is removed, as there is only a singlecoherent radar.

At step 522, the target map is generated to identify the range, Doppler,and angle values for each detected target. In selected embodiments, theradar controller processor may be configured to produce map dataidentifying paired range (r), Doppler ({dot over (r)}) and angle (θ)values for each detected/target object.

As disclosed herein, selected embodiments of the disclosed distributedaperture radar system may provide several enhancements when comparedwith conventional radar systems. In addition to enabling theconstruction of a single large coherent aperture from two or moredistributed radars which achieves high angular resolution and suppressesspurious side lobes, the disclosed distributed aperture radar system canuse RF front-end and signal processing blocks of existing radar designswithout modifications, thereby minimizing the cost of developing the newsolution. In addition, the distributed aperture radar system disclosedherein eliminates the requirement of physically sharing the LO signalsat each radar device by providing an efficient mechanism for usingestimated frequency and phase offset values to correlate the time,frequency and phase of distributed radar devices so that bi-static radarprinciples can be used to form a coherent aperture across a distance viaeavesdropping the other radar's transmission. In addition, the presentdisclosure eliminates the need to perform cross-correlation or matchedfiltering since only specified mixing and fast-time and slow-timeprocessing steps are required for estimating the frequency and phaseoffsets between radars. For systems that operate on independent, lowcost oscillators, the ability to efficiently derive the frequency offsetvalues is most helpful. In addition, when distributed radars areseparated at a distance, the formed apertures are typically sparse innature, resulting in spurious sidelobes, so the ability to mitigate orsuppress the side lobes as disclosed herein enables larger separationbetween the radars without significantly increasing false detections.

By now it should be appreciated that there has been provided adistributed bi-static radar architecture, circuit, method, and systemhaving first and second small aperture radar devices that are physicallydistributed from one another. In selected embodiments, the first andsecond small aperture radar devices each comprise a system-on-a-chip(SOC). In other embodiments, the first small aperture radar deviceincludes a first local oscillator reference clock generator, and thesecond small aperture radar device includes a second, independent localoscillator reference clock generator. The first and second smallaperture radar devices are connected to a radar control processing unitthat is configured to alternately select the first and second smallaperture radar devices to operate as the master unit so that radarsignals are sequentially transmitted from every transmit antenna in thefirst and second plurality of transmit and receive antennas. The firstsmall aperture radar device includes a first plurality of transmit andreceive antennas coupled to a first signal processor component that isconfigured to transmit a first radar signal from at least a firsttransmit antenna and to process target returns from the first smallaperture radar device to thereby generate a first mono-static virtualarray aperture when the first small aperture radar device is selected tooperate as a master unit, and that is configured to process targetreturns and an eavesdropped signal from the second small aperture radardevice received at one or more receive antennas of the first smallaperture radar device to generate a first bi-static virtual arrayaperture when the first small aperture radar device is selected tooperate as a slave unit. In similar fashion, the second small apertureradar device includes a second plurality of transmit and receiveantennas and a second signal processor component that is configured totransmit a second radar signal from at least a second transmit antennaand to process target returns from the second small aperture radardevice to thereby generate a second mono-static virtual array aperturewhen the second small aperture radar device is selected to operate as amaster unit, and that is configured to process target returns and aneavesdropped signal from the first small aperture radar device receivedat one or more receive antennas of the second small aperture radardevice to generate a second bi-static virtual array aperture when thesecond small aperture radar device is selected to operate as a slaveunit. The radar control processing unit is operable to produce targetscene information by coherently combining virtual array apertures fromthe first and second small aperture radar devices to construct anextended bi-static MIMO virtual array aperture that is larger than thefirst or second bi-static MIMO virtual array apertures produced,respectively, by only the first small aperture radar device or secondsmall aperture radar device. When operating as a slave unit, the secondsmall aperture radar device is configured to process target returns andan eavesdropped signal from the first small aperture radar device whenselected to operate as the master unit with fast and slow timeprocessing steps; to compute an estimated frequency offset and anestimated phase offset between the first and second small aperture radardevices based on a range-doppler map derived from the fast and slow timeprocessing steps; and to apply the estimated frequency offset andestimated phase offset to generate a bi-static virtual array aperture atthe second small aperture radar device that is coherent in frequency andphase with the mono-static virtual array aperture generated by the firstsmall aperture radar device selected to operate as the master unit. Whenoperating as a slave unit, the second small aperture radar devicecomputes the estimated frequency offset from the range-doppler map basedon doppler and range positions of an identified eavesdropped signal peakin the range-doppler map. In addition or in the alternative, the (slave)second small aperture radar device may compute the estimated phaseoffset from the range-doppler map based on a detected phase of a peakamplitude of an identified eavesdropped signal peak in the range-dopplermap. In addition or in the alternative, the second signal processorcomponent in the (slave) second small aperture radar device may computerange fast Fourier transforms (FFTs) and doppler FFTs on target returnsignals received at the one or more receive antennas of the second smallaperture radar device to generate the range-doppler map.

In another form, there is provided a method, system, and apparatus foroperating a distributed aperture bi-static radar system comprising firstand second small aperture radar devices that are physically distributedfrom one another and connected to a radar control processing unit. Inselected embodiments, the first small aperture radar device includes afirst local oscillator reference clock generator, and the second smallaperture radar device includes a second, independent local oscillatorreference clock generator. In the disclosed methodology, the radarcontrol processing unit selects the first small aperture radar device tooperate as a master unit and the second small aperture radar device tooperate as a slave unit. Thereafter, the first (master) small apertureradar device sequentially transmits a first radar signal from everytransmit antenna in a first plurality of transmit antennas at the firstsmall aperture radar device. In addition, the first (master) smallaperture radar device processes target returns received at one or morereceive antennas of the first small aperture radar device in response tothe first radar signal to generate a first mono-static virtual arrayaperture. In addition, the second (slave) small aperture radar deviceprocesses target returns and an eavesdropped signal from the first smallaperture radar device received at one or more receive antennas of thesecond small aperture radar device in response to the first radar signalto generate a first bi-static virtual array aperture. Subsequently, theradar control processing unit selects the second small aperture radardevice to operate as a master unit and the first small aperture radardevice to operate as a slave unit. Thereafter the second (master) smallaperture radar device sequentially transmits second radar signal fromevery transmit antenna in a second plurality of transmit antennas at thesecond small aperture radar device. In addition, the second (master)small aperture radar device processes target returns received at one ormore receive antennas of the second small aperture radar device inresponse to the second radar signal to generate a second mono-staticvirtual array aperture. Finally, the radar control processing unitproduces target scene information by coherently combining the first andsecond mono-static virtual array apertures and the first and secondbi-static virtual array apertures from the first and second distributedradar devices to construct an extended bi-static MIMO virtual arrayaperture that is larger than the first or second bi-static MIMO virtualarray apertures produced, respectively, by only the second smallaperture radar device or first small aperture radar device. In operationwhen operating as a slave unit, the second small aperture radar deviceprocesses target returns and an eavesdropped signal from the first smallaperture radar device when selected to operate as the master unit withfast and slow time processing steps; computes an estimated frequencyoffset and an estimated phase offset between the first and second smallaperture radar devices based on a range-doppler map derived from thefast and slow time processing steps; and applies the estimated frequencyoffset and estimated phase offset to generate a bi-static virtual arrayaperture at the second small aperture radar device that is coherent infrequency and phase with the mono-static virtual array aperturegenerated by the first small aperture radar device selected to operateas the master unit. In such embodiments, the second (slave) smallaperture radar device may compute the estimated frequency offset basedon doppler and range positions of an identified eavesdropped signal peakin the range-doppler map. In addition or in the alternative, the second(slave) small aperture radar device may compute the estimated phaseoffset based on a detected phase of a peak amplitude of an identifiedeavesdropped signal peak in the range-doppler map. In addition or in thealternative, the second (slave) small aperture radar device may computerange fast Fourier transforms (FFTs) and doppler FFTs on target returnsignals received at the one or more receive antennas of the second smallaperture radar device to generate the range-doppler map.

In yet another form, there is provided a distributed aperture bi-staticradar system, method, and apparatus which includes a first and secondmulti-antenna radar devices that are physically distributed from oneanother and that may each be embodied as a system-on-a-chip (SOC). Inparticular, the first multi-antenna radar device includes a firstplurality of transmit and receive antennas coupled to a first signalprocessor component. In addition, the second multi-antenna radar deviceincludes a second plurality of transmit and receive antennas coupled toa second signal processor component. In selected embodiments, the firstmulti-antenna radar device includes a first local oscillator referenceclock generator, and the second multi-antenna radar device includes asecond, independent local oscillator reference clock generator. In thedistributed aperture bi-static radar system, the first and secondmulti-antenna radar devices are coupled to a radar control processingunit that is configured to alternately select the first and secondmulti-antenna radar devices to operate as the master unit so that radarsignals are sequentially transmitted from every transmit antenna in thefirst and second plurality of transmit and receive antennas. Thusconnected, the first multi-antenna radar device is configured totransmit a first radar signal from at least a first transmit antenna andto process target returns from the first multi-antenna radar device tothereby generate a first mono-static virtual array aperture when thefirst multi-antenna radar device is selected to operate as a masterunit, and that is configured to process target returns and aneavesdropped signal from the second multi-antenna radar device receivedat one or more receive antennas of the first multi-antenna radar deviceto generate a first bi-static virtual array aperture when the firstmulti-antenna radar device is selected to operate as a slave unit. Inaddition, the second multi-antenna radar device is configured totransmit a second radar signal from at least a second transmit antennaand to process target returns from the second multi-antenna radar deviceto thereby generate a second mono-static virtual array aperture when thesecond multi-antenna radar device is selected to operate as a masterunit, and that is configured to process target returns and aneavesdropped signal from the first multi-antenna radar device receivedat one or more receive antennas of the second multi-antenna radar deviceto generate a second bi-static virtual array aperture when the secondmulti-antenna radar device is selected to operate as a slave unit. Whenoperating as a slave unit, the second multi-antenna device may beconfigured to process target returns and an eavesdropped signal from thefirst multi-antenna radar device when selected to operate as the masterunit with fast and slow time processing steps, to compute an estimatedfrequency offset and an estimated phase offset between the first andsecond multi-antenna radar devices based on a range-doppler map derivedfrom the fast and slow time processing steps, and to apply the estimatedfrequency offset and estimated phase offset to generate a bi-staticvirtual array aperture at the second multi-antenna radar device that iscoherent in frequency and phase with the mono-static virtual arrayaperture generated by the first multi-antenna radar device selected tooperate as the master unit. In such embodiments, the second (slave)multi-antenna radar device may compute the estimated frequency offsetbased on doppler and range positions of an identified eavesdroppedsignal peak in the range-doppler map. In addition or in the alternative,the second (slave) multi-antenna radar device may compute the estimatedphase offset based on a detected phase of a peak amplitude of anidentified eavesdropped signal peak in the range-doppler map. Inaddition or in the alternative, the second (slave) multi-antenna radardevice may compute range fast Fourier transforms (FFTs) and doppler FFTson target return signals received at the one or more receive antennas ofthe second small aperture radar device to generate the range-dopplermap. With this arrangement, the radar control processing unit isconfigured to produce target scene information by coherently combiningvirtual array apertures from the first and second distributed radardevices to construct an extended bi-static MIMO virtual array aperturethat is larger than the first or second bi-static MIMO virtual arrayapertures produced, respectively, by only the first distributed radardevice or second distributed radar device.

Although the described exemplary embodiments disclosed herein focus onexample automotive radar circuits, systems, and methods for using same,the present invention is not necessarily limited to the exampleembodiments illustrate herein. For example, various embodiments of adistributed aperture radar may be applied in non-automotiveapplications, and may use additional or fewer circuit components thanthose specifically set forth. Thus, the particular embodiments disclosedabove are illustrative only and should not be taken as limitations uponthe present invention, as the invention may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. Accordingly, the foregoingdescription is not intended to limit the invention to the particularform set forth, but on the contrary, is intended to cover suchalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claimsso that those skilled in the art should understand that they can makevarious changes, substitutions and alterations without departing fromthe spirit and scope of the invention in its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

What is claimed is:
 1. A distributed aperture bi-static radar systemcomprising first and second small aperture radar devices that arephysically distributed from one another and connected to a radar controlprocessing unit, the first small aperture radar device comprising afirst plurality of transmit and receive antennas coupled to a firstsignal processor component that is configured to transmit a first radarsignal from at least a first transmit antenna and to process targetreturns from the first small aperture radar device to thereby generate afirst mono-static virtual array aperture when the first small apertureradar device is selected to operate as a master unit, and that isconfigured to process target returns and an eavesdropped signal from thesecond small aperture radar device received at one or more receiveantennas of the first small aperture radar device to generate a firstbi-static virtual array aperture when the first small aperture radardevice is selected to operate as a slave unit; the second small apertureradar device that is physically distributed from the first smallaperture radar device comprising a second plurality of transmit andreceive antennas and a second signal processor component that isconfigured to transmit a second radar signal from at least a secondtransmit antenna and to process target returns from the second smallaperture radar device to thereby generate a second mono-static virtualarray aperture when the second small aperture radar device is selectedto operate as a master unit, and that is configured to process targetreturns and an eavesdropped signal from the first small aperture radardevice received at one or more receive antennas of the second smallaperture radar device to generate a second bi-static virtual arrayaperture when the second small aperture radar device is selected tooperate as a slave unit; and the radar control processing unitconfigured to alternately select the first and second small aperturedevices to operate as the master unit so that radar signals aresequentially transmitted from every transmit antenna in the first andsecond plurality of transmit and receive antennas, the radar controlprocessing unit operable to produce target scene information bycoherently combining virtual array apertures from the first and secondsmall aperture radar devices to construct an extended bi-static MIMOvirtual array aperture that is larger than the first or second bi-staticMIMO virtual array apertures produced, respectively, by only the firstsmall aperture radar device or second small aperture radar device. 2.The distributed aperture radar system of claim 1, where the first andsecond small aperture radar devices each comprise a system-on-a-chip(SOC).
 3. The distributed aperture radar system of claim 1, where thefirst small aperture radar device comprises a first local oscillatorreference clock generator, and where the second small aperture radardevice comprises a second, independent local oscillator reference clockgenerator.
 4. The distributed aperture radar system of claim 1, wherethe second small aperture radar device, when operating as a slave unit,is configured to: process target returns and an eavesdropped signal fromthe first small aperture radar device when selected to operate as themaster unit with fast and slow time processing steps, compute anestimated frequency offset and an estimated phase offset between thefirst and second small aperture radar devices based on a range-dopplermap derived from the fast and slow time processing steps, and apply theestimated frequency offset and estimated phase offset to generate abi-static virtual array aperture at the second small aperture radardevice that is coherent in frequency and phase with the mono-staticvirtual array aperture generated by the first small aperture radardevice selected to operate as the master unit.
 5. The distributedaperture radar system of claim 4, where the second small aperture radardevice computes the estimated frequency offset from the range-dopplermap based on doppler and range positions of an identified eavesdroppedsignal peak in the range-doppler map.
 6. The distributed aperture radarsystem of claim 4, where the second small aperture radar device computesthe estimated phase offset from the range-doppler map based on adetected phase of a peak amplitude of an identified eavesdropped signalpeak in the range-doppler map.
 7. The distributed aperture radar systemof claim 4, where the second signal processor component computes rangefast Fourier transforms (FFTs) and doppler FFTs on target return signalsreceived at the one or more receive antennas of the second smallaperture radar device to generate the range-doppler map.
 8. A method foroperating a distributed aperture bi-static radar system comprising firstand second small aperture radar devices that are physically distributedfrom one another and connected to a radar control processing unit,comprising: selecting, by radar control processing unit, the first smallaperture radar device to operate as a master unit and the second smallaperture radar device to operate as a slave unit; sequentiallytransmitting a first radar signal from every transmit antenna in a firstplurality of transmit antennas at the first small aperture radar device;processing, at the first small aperture radar device, target returnsreceived at one or more receive antennas of the first small apertureradar device in response to the first radar signal to generate a firstmono-static virtual array aperture; processing, at the second smallaperture radar device, target returns and an eavesdropped signal fromthe first small aperture radar device received at one or more receiveantennas of the second small aperture radar device in response to thefirst radar signal to generate a first bi-static virtual array aperture;then selecting, by radar control processing unit, the second smallaperture radar device to operate as a master unit and the first smallaperture radar device to operate as a slave unit; sequentiallytransmitting a second radar signal from every transmit antenna in asecond plurality of transmit antennas at the second small aperture radardevice; processing, at the second small aperture radar device, targetreturns received at one or more receive antennas of the second smallaperture radar device in response to the second radar signal to generatea second mono-static virtual array aperture; and then producing targetscene information at the radar control processing unit by coherentlycombining the first and second mono-static virtual array apertures andthe first and second bi-static virtual array apertures from the firstand second distributed radar devices to construct an extended bi-staticMIMO virtual array aperture that is larger than the first or secondbi-static MIMO virtual array apertures produced, respectively, by onlythe second small aperture radar device or first small aperture radardevice.
 9. The method of claim 8, where the first small aperture radardevice comprises a first local oscillator reference clock generator, andwhere the second small aperture radar device comprises a second,independent local oscillator reference clock generator.
 10. The methodof claim 8, where the second small aperture radar device, when operatingas a slave unit: processes target returns and an eavesdropped signalfrom the first small aperture radar device when selected to operate asthe master unit with fast and slow time processing steps; computes anestimated frequency offset and an estimated phase offset between thefirst and second small aperture radar devices based on a range-dopplermap derived from the fast and slow time processing steps; and appliesthe estimated frequency offset and estimated phase offset to generate abi-static virtual array aperture at the second small aperture radardevice that is coherent in frequency and phase with the mono-staticvirtual array aperture generated by the first small aperture radardevice selected to operate as the master unit.
 11. The method of claim10, where the second small aperture radar device computes the estimatedfrequency offset based on doppler and range positions of an identifiedeavesdropped signal peak in the range-doppler map.
 12. The method ofclaim 10, where the second small aperture radar device computes theestimated phase offset based on a detected phase of a peak amplitude ofan identified eavesdropped signal peak in the range-doppler map.
 13. Themethod of claim 10, where a second signal processor component in thesecond small aperture radar device computes range fast Fouriertransforms (FFTs) and doppler FFTs on target return signals received atthe one or more receive antennas of the second small aperture radardevice to generate the range-doppler map.
 14. A distributed aperturebi-static radar system, comprising: a first multi-antenna radar devicecomprising a first plurality of transmit and receive antennas coupled toa first signal processor component; a second multi-antenna radar devicethat is physically distributed from the first multi-antenna radar deviceand that comprises a second plurality of transmit and receive antennascoupled to a second signal processor component; and a radar controlprocessing unit configured to alternately select the first and secondmulti-antenna radar devices to operate as the master unit so that radarsignals are sequentially transmitted from every transmit antenna in thefirst and second plurality of transmit and receive antennas, wherein thefirst multi-antenna radar device is configured to transmit a first radarsignal from at least a first transmit antenna and to process targetreturns from the first multi-antenna radar device to thereby generate afirst mono-static virtual array aperture when the first multi-antennaradar device is selected to operate as a master unit, and that isconfigured to process target returns and an eavesdropped signal from thesecond multi-antenna radar device received at one or more receiveantennas of the first multi-antenna radar device to generate a firstbi-static virtual array aperture when the first multi-antenna radardevice is selected to operate as a slave unit, wherein the secondmulti-antenna radar device is configured to transmit a second radarsignal from at least a second transmit antenna and to process targetreturns from the second multi-antenna radar device to thereby generate asecond mono-static virtual array aperture when the second multi-antennaradar device is selected to operate as a master unit, and that isconfigured to process target returns and an eavesdropped signal from thefirst multi-antenna radar device received at one or more receiveantennas of the second multi-antenna radar device to generate a secondbi-static virtual array aperture when the second multi-antenna radardevice is selected to operate as a slave unit, and wherein the radarcontrol processing unit is configured to produce target sceneinformation by coherently combining virtual array apertures from thefirst and second distributed radar devices to construct an extendedbi-static MIMO virtual array aperture that is larger than the first orsecond bi-static MIMO virtual array apertures produced, respectively, byonly the first distributed radar device or second distributed radardevice.
 15. The distributed aperture bi-static radar system of claim 14,where the first and second multi-antenna radar devices each comprise asystem-on-a-chip (SOC).
 16. The distributed aperture bi-static radarsystem of claim 14, where the first multi-antenna radar device comprisesa first local oscillator reference clock generator, and where the secondmulti-antenna radar device comprises a second, independent localoscillator reference clock generator.
 17. The distributed aperturebi-static radar system of claim 14, where the second multi-antennadevice, when operating as a slave unit, is configured to: process targetreturns and an eavesdropped signal from the first multi-antenna radardevice when selected to operate as the master unit with fast and slowtime processing steps, compute an estimated frequency offset and anestimated phase offset between the first and second multi-antenna radardevices based on a range-doppler map derived from the fast and slow timeprocessing steps, and apply the estimated frequency offset and estimatedphase offset to generate a bi-static virtual array aperture at thesecond multi-antenna radar device that is coherent in frequency andphase with the mono-static virtual array aperture generated by the firstmulti-antenna radar device selected to operate as the master unit. 18.The distributed aperture bi-static radar system of claim 17, where thesecond multi-antenna radar device computes the estimated frequencyoffset based on doppler and range positions of an identifiedeavesdropped signal peak in the range-doppler map.
 19. The distributedaperture bi-static radar system of claim 17, where the secondmulti-antenna radar device computes the estimated phase offset based ona detected phase of a peak amplitude of an identified eavesdroppedsignal peak in the range-doppler map.
 20. The distributed aperturebi-static radar system of claim 18, where the second multi-antenna radardevice computes range fast Fourier transforms (FFTs) and doppler FFTs ontarget return signals received at the one or more receive antennas ofthe second small aperture radar device to generate the range-dopplermap.